The lungs are made up of millions of tiny balloons
called alveoli, which fill with air each time you
inhale and deflate each time you exhale. The many
microscopic alveoli make the lungs look like sponges.
Alveoli are made up of squamous epitheliel tissue,
which is very thin and elastic, like the walls of a
balloon.

When you breath, air goes down your windpipe or
trachea. The trachea branches off to either lung;
these branches are called branchi. Each branchus
branches again and again into smaller and smaller
branches, like a tree. The tiniest of these branches
are called bronchioles. Each bronchiole supplies air
to a cluster of alveoli, called an alveolar sac. The
individual alveoli are connected to the bronchioles by
tiny ducts.

Each alveolar sac is surrouned by a capillary bed,
which is a network of tiny blood vessels. The walls
of the blood vessels and the balloon-like walls of the
alveoli are so thin that oxygen molecules
(O2) can pass from the air-filled alveoli
to red blood cells inside the vessels. Likewise,
carbon dioxide molecules (CO2) can pass
from the red blood cells into the alveoli. When the
body exhales, the CO2 is removed.

The lungs are housed in the pleural cavity. The
pleural cavity is created by the rib cage, which
surrounds and protects the lungs, and by the
diaphragm, a thin, flat muscle which separates the
heart and lungs from the organs of the abdomen like a
sheet. When the diaphragm is relaxed, it balloons
upward. When the diaphragm contracts, it flattens out
and pushes downward. When the muscles of the rib cage
contract, they spread and raises the ribs. When both
the diaphram and the muscles of the rib cage contract
at the same time, the pleural cavity becomes larger
and a vacuum is created which sucks air into the
lungs. This process is called inhalation. When the
rib muscles relax, the ribcage lowers and comes back
together. At the same time, the diaphragm relaxes.
The space inside the pleural cavity becomes smaller
and the air is pushed back out of the lungs. This
process is called exhalation.

Cellular Respiration

The lungs are responsible for bringing oxygen into
the body and taking carbon dioxide out. Every cell in
the body needs oxygen to live and function. Oxygen is
used in many of the cell's chemical reactions. The
most important of these chemical processes is the
breakdown of glucose (simple sugar) to produce the
energy needed to keep the cell alive and allow it to
perform its special tasks. Carbon dioxide, on the
other hand, is a waste product of energy. If carbon
dioxide was allowed to build up, it would poison the
cell and eventually the entire body.

Oxygen is brought from the lungs to individual
cells by the bloodstream. Oxygen passes from the
alveoli in the lungs into the surrounding capillaries
and attaches to the hemaglobin found in red blood
cells. Hemaglobin is a chemical, found in red blood
cells, which can bind with oxygen and carbon dioxide,
but it loves oxygen and will readily trade the
CO2 it has carried back from the cells for
an oxygen molecule. When hemaglobin is carrying
oxygen, it is red; when it is carrying CO2,
it is blue.

The lungs are placed near the heart so that when
blood returns from the rest of the body full of
CO2, it can be pumped directly to the lungs
to trade its CO2 for O2. It
then goes back to the heart, full of O2, to
be pump back into the rest of the body and its
individual cells.